1. Field of the Invention
The present invention relates to computing devices coupled together into a wireless local area network, and more particularly, to a wireless local area network infrastructure that permits communication in plural modes to support both wideband spread spectrum and narrowband radio frequency signals.
2. Description of Related Art
A wireless local area network (WLAN) comprises a plurality of remote computing devices which communicate together over radio frequency (RF) signals. As in a wired local area network (LAN), the WLAN allows users to seamlessly access disk drives, printers, and additional computer resources and systems connected to the WLAN. The remote computing devices include a radio receiver/transmitter adapted for RF communication with the other elements of the WLAN. The WLAN may also include a central host processing unit that sends information to and receives information from any one of the plurality of remotely disposed computing devices. The central host processor may also form part of a separate wired LAN to provide a bridge with the WLAN. In such a WLAN, the remote computing devices may comprise portable units that operate within a defined environment to report information back to the central host processing unit. WLAN systems offer increased flexibility over wired LAN systems by enabling operators of the remote computing devices substantial freedom of movement through the environment, and are particularly useful for remote data collection applications such as inventory control, manufacturing and production flow management, and asset tracking.
For simplicity, the radio receiver/transmitter provided within each remote computing device may communicate using conventional narrowband RF signals. Narrowband RF operation has a significant drawback in that the radio receiver/transmitter must be operated at relatively low power levels in order to ensure compliance with certain governmental regulations, and at such low power levels the RF signals are highly susceptible to interference and have low data throughput rates. To overcome these and other drawbacks, commercial WLAN systems have adopted so-called "spread spectrum" modulation techniques. In a spread spectrum system, the transmitted signal is spread over a frequency band that is significantly wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby reducing interference to other radio devices.
In one type of spread spectrum communication system, an RF carrier is shifted in discrete increments in a pattern dictated by a predetermined sequence. These spread spectrum systems are known as "frequency-hopping" modulation systems, since the transmitter jumps from frequency to frequency in accordance with the predetermined sequence. The information signal is modulated onto the shifting carrier frequencies using frequency shift keying (FSK) modulation. Another type of spread spectrum communication system utilizes an RF carrier modulated by a digital code sequence having a spreading code rate, or chipping rate, much higher than the clock rate of the information signal. These spread spectrum systems are known as "direct sequence" modulation systems. The RF carrier may be modulated such that a data stream has one phase when a spreading code sequence represents a data "one" and 180.degree. phase shift when the spreading code sequence represents a data "zero." The RF carrier may also be binary or quadrature modulated by one or more data streams such that the data streams have one phase when a spreading code sequence represents a data "one" and a predetermined phase shift (e.g., 180.degree. for binary, and 90.degree. for quadrature) when the spreading code sequence represents a data "zero." These types of modulation are commonly referred to as binary shift key (BPSK) and quadrature shift key (QPSK) modulation, respectively.
A primary drawback of operating a WLAN using spread spectrum communication is the high cost of the computing devices due primarily to the complexity of the radio receiver/transmitter. For certain applications, a narrowband RF radio receiver/transmitter would provide satisfactory performance while the high data throughput and integrity provided by a wideband spread spectrum radio receiver/transmitter would be unnecessary. Nevertheless, it would be costly and impractical to operate two separate narrowband and wideband WLAN systems simultaneously. As a result, WLAN system designers must select a single communication mode that provides a sufficient level of performance within practical cost parameters.
Thus, it would be highly desirable to provide a WLAN infrastructure that permits multi-mode communication over both wideband spread spectrum and narrowband RF signals. Such a multi-mode WLAN could be constructed using a combination of higher performance computing devices communicating using wideband spread spectrum RF signals and lower performance computing devices communicating using narrowband RF signals.